El Niño and La Niña| Earth systems and resources| AP Environmental science| Khan Academy

Every few years, you might hear about El Niño in the news, and this also might come with powerful images of flooding and rainfall. But El Niño is not just a storm; it's actually a climate pattern that takes place in the Pacific Ocean, and we'll get a little bit more into what that actually means.

Now, a fun fact about how El Niño was named is that a long time ago, South American fishermen noticed one December that the Pacific Ocean was actually warmer than it normally is, and this brought about an abundance of fish. So, because they were grateful, they named this event El Niño to correlate with the commemoration of Christ in this part of the world during Christmas time. So, that's a little trivia you can keep in your back pocket.

But back to what we hear about El Niño in the news; you might also notice that it isn't talked about every single year. That's because El Niño comes around every two to seven years on average, and scientists are still not sure exactly what triggers El Niño. But they know what signs to look for once it is approaching or once we're in that event.

So, even though it's true that El Niño can bring about heavy rainfall and flooding, it can also cause severe drought. It's really important to note that different regions around the world experience different effects of El Niño, and we'll see a few examples of those. Sometimes an El Niño year is actually a little bit helpful and might bring about some much-needed rain, but other times, bigger and more severe El Niño events can bring devastating weather events all across the globe.

So, we can look at the different effects by looking at the biggest El Niño on record during the 1997-1998 season, and we can see how different regions were affected. For example, in California, we saw very destructive mudslides. In Ecuador, there was heavy rainfall, and in Indonesia, there was actually extreme drought and fire. Overall, this El Niño event was really large and very destructive. There are estimates of about 36 billion dollars in damage to infrastructure.

So, you might be asking yourself, how can the same climate pattern cause such drastically different effects around the globe? To understand this better, we have to look at what a "normal" year looks like. So, this is a map of the Pacific Ocean, and normally there are trade winds that are pushing warm water towards the west, so towards Asia and the Pacific Islands. Warm water accumulates on this side, while on the other side, on the east side near Central and South America, we have an accumulation of cool water. This is also due to a process called upwelling, where cold water from the bottom of the ocean is pushed up.

During these conditions, in general, you would see less rain on this side, while you'll see more rain on this side.

Okay, so now let's look at what changes during an El Niño year. During an El Niño year, you still have trade winds pushing water west, but they're much weaker. That's the key here: your trade winds are weakened, which means less water is pushed towards Asia and the Pacific Islands. Compared to a normal year, you actually have cooler water that starts to accumulate on this side of the Pacific Ocean. Meanwhile, on the other side, since less water is being pushed away from Central and South America, you get more warm water accumulating.

So now you can think back to those South American fishermen and why they were experiencing warm water along their coasts during an El Niño year. This map does a really great job of showing the temperature gradient and how this warm water is staying along the coast of Central and South America during an El Niño year. Meanwhile, you have cold water accumulating towards Asia, and this causes a lot of changes that affect weather patterns.

In the U.S., the Pacific jet stream moves south of its neutral position, and because of that, we start to see that the northern U.S. and Canada are actually warmer and drier than they usually are. The U.S. Gulf Coast and Southwest regions of the U.S. are wetter and at a more increased risk for flooding, and South Asia and the Pacific Islands experience warmer and drier conditions that lead to drought on the other side of the Pacific Ocean.

So now you start to see how one event, El Niño, this one climate pattern can cause both flooding on one side of the Pacific Ocean while also causing drought on the other side. Finally, we'll end with La Niña because you might hear this come up too when talking about El Niño.

So, La Niña is essentially the opposite. If you've got El Niño down, you can start to understand what happens during a La Niña event. In this situation, our trade winds are getting stronger than our normal years, so they're pushing water west but they're doing so even more than we normally would expect. This warm water that accumulates towards Asia actually accumulates more west than it would during our normal years, and on the other side, we have even more upwelling causing this cold water to accumulate.

So, the next time you hear El Niño or La Niña in the news, you'll know that they're not just talking about a really big storm, but they're actually talking about climate patterns that can affect weather all across the globe.